The invention discloses a dual-frequency dual-mode gyrotron traveling wave tube input coupler, which relates to the technical field of microwave and millimeter wave devices, in particular to a gyrotron traveling wave tube input coupler. The dual-frequency dual-mode gyrotron traveling wave tube input coupler is implemented by adopting the technical measures that the structure of the dual-band (Ku/Ka-band) dual-working-mode (TE11/TE01-mode) input coupler comprises three main parts including a TE11-mode input coupler operating in the Ku band, a TE01-mode input coupler operating in the Ka band, and a cascade reflection cavity structure having a mode selecting property, thereby realizing the purpose of the invention. Compared with the traditional input coupler, the input coupler disclosed by the invention can operate both in the Ku and the Ka band; by adopting the cascade reflection cavity structure, thus total reflection of the TE01 mode is realized, and the single-direction output of theTE10 mode by means of an output circular waveguide is guaranteed; and by adopting a vane loading attenuation material, a competitive mode of attenuating the TE01 mode is achieved.